Regulatory Role for Neuregulin-1—Reining in LTP via Dopamine D4 Receptor

6 October 2008. Since genetic studies began to show an association between schizophrenia and the gene for neuregulin-1 (NRG-1), interest in the cell surface signaling properties of the protein has grown among researchers studying psychoses and related disorders. But while the strength of the genetic association between NRG-1 and schizophrenia may be debatable, what is perhaps even less clear is how the protein might fit with the major pathological characteristics of the disease. Some new hints emerged in last week’s PNAS online. Researchers led by Andres Buonanno at the National Institute of Child Health and Development, Bethesda, Maryland, reported that NRG-1 attenuates glutamatergic signaling through its actions on D2-like dopamine receptors. In one fell swoop the study connects neuregulin-1 to two major hypotheses of schizophrenia, namely that the disease is caused by disruption to glutamatergic and/or dopaminergic circuits.

Buonanno and colleagues previously showed that NRG-1 attenuates long-term potentiation in the CA1 field of the hippocampus (see Kwon et al., 2005). Long-term potentiation (LTP), or increased sensitivity to neuronal stimulation, requires an elaborate interplay between different classes of glutamate receptors and is crucial for learning and memory. Any disruption to LTP, particularly in the hippocampus, might at least partly explain the cognitive challenges faced by schizophrenia patients. Now, in their latest paper, first author Oh Bin Kwon and colleagues show that neuregulin’s effects on hippocampal LTP are mediated by activation of the D4 dopamine receptor, a member of the D2 receptor subfamily (D1 and D5 receptors are members of the D1 family, while D2, D3 and D4 receptors are part of the same subfamily). D2 antagonists are currently in widespread use as atypical antipsychotics.

To probe the potential effects of NRG-1 in the hippocampus, Kwon and colleagues used reverse microdialysis to inject a whiff of NRG-1β (in this case a fragment of the 1β isoform containing the ErbB4 ligand domain) into the CA1 region of rats. They found that the neuregulin boosted extracellular dopamine (DA) levels by about threefold (measured by microdialysis). This DA increase lasted about 12 minutes and appears to be caused by stimulated release of dopamine from neurons since blocked reuptake or metabolism could not explain the results: the authors found that this area of the brain showed no immunoreactivity for the dopamine reuptake transporter DAT, for example, and that production of homovanillic acid, a major DA metabolite, was actually elevated, by nearly fourfold, after infusing NRG-1β.

Could the boost in DA release be linked to NRG-1’s attenuation of LTP? To test this idea the authors measured NRG-1β blockage of LTP in hippocampal slices while perfusing them with dopamine receptor inhibitors. They found that the D1/D5 blocker SCH39166 had no effect. As for the D2 family, the D2/D3 antagonist sulpiride, which has 100-fold weaker affinity for the D4 receptor, likewise had no effect. However, the D4-specific antagonist L-745,870 completely blocked the attenuation of LTP by NRG-1β. Clozapine, which binds the D2 family and is currently considered the most effective schizophrenia treatment, also relieved the suppression of LTP. The results suggest that dopaminergic responses are intimately tied in with NRG-1β’s suppression of LTP and that it is the D4 receptor, specifically, that mediated the effects.

The authors confirmed this scenario using independent experimental models. In mice that have the D4 receptor knocked out, NRG-1β failed to depotentiate LTP as did sub-threshold theta pulse stimulation, which normally suppresses LTP. Also, activating D4 receptors directly had the same effect on LTP as adding NRG-1β—the specific D4 agonist PD168077 depotentiated LTP (an effect that was blocked by L-745,870, the D4 blocker), while D2/3 agonists had no effect.

How does activation of the D4 receptor by NRG-1β tone down LTP? To address this, the authors looked at currents evoked by activating the AMPA and NMDA glutamate receptor subtypes. During LTP, AMPA receptors on the surface of the postsynaptic membrane are reinforced by additional receptors, leading to increased AMPA currents. But D4 receptor activation reduced AMPA currents, while leaving NMDA currents intact. D4 activation also led to the uptake into the cell of the glutamate receptor 1 subunit of the AMPA receptor from the postsynaptic membrane, suggesting that AMPA receptors are actively removed from the synapse. All told, the findings suggest that NRG-1β boosts dopaminergic output leading to a specific activation of the D4 receptor, a loss of postsynaptic AMPA receptors, and thus an attenuation of LTP. “These results introduce two important novel concepts: (1) they demonstrate how NRG-1 functionally links dopaminergic and glutamatergic transmission, and (2) they show that NRG-1 can attenuate LTP via D4Rs,” write the authors.

As for where this whole cascade starts, it may involve yet another type of transmission that has been linked to schizophrenia. The authors note that NRG-1β is unlikely to act directly on dopaminergic neurons since they lack ErbB4, the main receptor for the neuregulin ligand. However, GABAergic neurons in the hippocampus and forebrain have the highest ErbB4 expression, “[t]hus, the most parsimonious explanation for the acute actions of NRG-1β on DA release is via a local hippocampal circuit involving GABAergic interneurons,” the authors write. A variety of genetic and pathological data also suggest that these interneurons are not functioning to capacity in schizophrenia (see SRF related news story).—Tom Fagan.

The evidence is becoming overwhelming that the GABA system disturbances are a critical hallmark of schizophrenia. The data indicate that these processes are present across different brain regions, albeit with some notable differences in the exact genes affected. Synthesizing the observations from the recent scientific reports strongly suggest that the observed GABA system disturbances arise as a result of complex genetic-epigenetic-environmental-adaptational events. While we currently do not understand the nature of these interactions, it is clear that this will become a major focus of translational neuroscience over the next several years, including dissecting the pathophysiology of these events using in vitro and in vivo experimental models.

The three papers discussed in the above News article are the most recent to imply dysregulation of the cortical GABAergic system in schizophrenia and related disease. Each paper adds a new twist to the story—molecular changes in the hippocampus of schizophrenia and bipolar subjects show striking differences dependent on layer and subregion (Benes et al), and in prefrontal cortex, there is mounting evidence that changes in the "GABA-transcriptome" affect certain subtypes of inhibitory interneurons (Hashimoto et al). The polymorphisms in the GAD1/GAD67 (GABA synthesis) gene which Straub el al. identified as genetic modifiers for cognitive performance and as schizophrenia risk factors will undoubtedly spur further interest in the field; it will be interesting to find out in future studies whether these genetic variants determine the longitudinal course/outcome of the disease, treatment response etc etc.